Could bats act as reservoir hosts for Rift Valley fever virus?

The inter-epizootic reservoir host of Rift Valley fever virus (RVFV) remains unknown, although the namaqua rock rat, Aethomys namaquensis, as well as bats have been implicated. Bats can be asymptomatically infected with rabies, as well as several arboviruses; the possibility that they can act as host for RVFV therefore exists. To examine this possibility, 350 different samples (brain, liver, salivary glands and brown fat) obtained from 150 bats (comprising seven species) were tested for RVFV antigen using an enzyme linked immunosorbent assay (ELISA). None of the samples tested positive, but the ELISA proved to have limited sensitivity (> or = 10(3) TCID50/ml). In order to determine whether bats could be infected with RVFV, one Miniopterus schreibersii and two Eptesicus capensis bats were inoculated by the oral or intramuscular route with 100 ml and 30 ml, respectively, of a RVFV suspension with a titre of 10(6) TCID50/ml. None of the bats developed any clinical signs. A low concentration of RVFV antigen was found in the liver and urine of M. schreibersii, but not in brain tissue. A third E. capensis bat was inoculated by the intramuscular route and sacrificed on day 18. A low level of antigen was detected in the brown fat. These results demonstrate that bats can be infected with RVFV, and that further studies should be done to determine the potential of different bat species to act as reservoir hosts for RVFV during inter-epizootic periods.

Rift Valley fever virus: a seroepidemiologic study of small terrestrial vertebrates in South Africa.

Epizootics of Rift Valley fever (RVF) are often associated with periods of heavy rainfall, which are favorable for mosquito vectors. However, in seasons with normal or low rainfall, enzootic circulation occurs, suggesting the existence of a natural host that can act as a cryptic carrier during interepizootic periods. To confirm the role of heavy rainfall in epizootic circulation, and to identify a possible natural host of RVF virus, serum samples from small terrestrial mammals in the Free State and Northern Cape regions of South Africa were collected before and after the 1988 floods. These areas are known to support epizootic circulation of RVF virus. The samples were tested for the presence of RVF virus-specific IgG using an ELISA and positive sera were confirmed by a neutralization test. Forty-seven (15%) of 312 Aethomys namaquensis (Namaqua rock rat) had antibodies to RVF virus. Of these positive sera, nine (6%) of 141 were collected before the floods of 1988 and 38 (22%) of 171 were collected afterwards (P = 0.001). Naive A. namaquensis were inoculated with RVF virus and developed a viremia, but no clinical symptoms, suggesting that they can act as temporary asymptomatic carriers of the virus. These results suggest a role for A. namaquensis as a cryptic carrier for RVF virus during interepizootic periods and support the results of other studies suggesting an amplifying role for heavy rainfall in the circulation of RVF virus.

The binding potential of commercial antibody conjugates with sera of various small terrestrial mammals.

Infectious diseases of wild animals are of increasing importance, both from an economic viewpoint and because several of these diseases are pathogenic to man. However, serosurveys to determine the circulation of infectious organisms in wildlife are complicated by the fact that antibodies to species-specific immunoglobulins are not available for use in serological assays such as enzyme-linked immunosorbent assays (ELISAs) or immunofluorescence assays. To determine the binding potential of four commercially available antibody conjugates with the sera of wild animals, sera from 27 species of small terrestrial mammals were allowed to react with alkaline phosphatase-labelled protein A, anti-rabbit IgG, anti-mouse IgG and anti-human IgG by by the use of an ELISA. It was found that sera from some species of the order Lagomorpha bound optimally to anti-rabbit IgG, while anti-mouse IgG could be used for most species of Rodentia. For all Carnivora, Insectivora, Macroscelidea, Hyracoidea and other Rodentia, staphylococcal protein A demonstrated optimal binding. None of the sera that was tested bound to anti-human IgG. These results demonstrate that commercial conjugates can be used in serological assays in which wild animal sera are used, and should be useful for future serosurveys to determine the circulation of infectious agents in small terrestrial mammals.

Rabies and bats in a rabies-endemic area of southern Africa: application of two commercial test kits for antigen and antibody detection.

In southern Africa, isolates of rabies-related viruses (i.e. Duvenhage virus and Lagos bat virus) have been made from insectivorous and frugivorous bats. As no recent formal bat virus survey has been reported in southern Africa, a survey of bats in rabies-endemic areas was undertaken. Five hundred and forty-seven bats (13 species) were collected from 21 localities in the Orange Free State, Lesotho and the northern Cape Province. None of the 190 bat sera tested using the "Trousse Platelia Rage" ELISA kit (Diagnostic Pasteur), had antibodies to rabies virus glycoprotein G. Rabies virus nucleocapsid antigen was also sought for in the brains of 530 bats (13 species) by means of the "Rapid rabies enzyme immunodiagnosis" (RREID) test (Diagnostics Pasteur). No positive results were obtained. These results show that bats are unlikely to play an important role as hosts of rabies in these parts of Africa, although a low rate of infection cannot be excluded.

Immunity to rabies after administration of prophylactic human diploid-cell vaccine.

The immune status to rabies of 14 volunteers was determined using the commercially available Trousse Platelia Rage (Diagnostics Pasteur) enzyme immunoassay test system. Twelve subjects were evaluated before and between 6 months and 60 months after prophylactic intramuscular (deltoid) administration of rabies human diploid-cell vaccine, while the effect of booster doses on a further 2 volunteers was evaluated over an 11-year period. Optical density values were converted to international units to allow correlation with World Health Organisation seroneutralisation references. Values of greater than or equal to 0.5 IU are considered protective. The results showed that most individuals were still immune 2 years after vaccination; there was a tendency for serum antibody levels to decrease over a 5-year post-vaccination period. Antibody levels rose sharply after booster immunisation, after which they decreased at a much slower rate. In general, results revealed that after the first booster, additional booster vaccinations at 5-yearly intervals would provide adequate prophylactic immunity. There was, however, much individual variation, which emphasises the need to evaluate each individual at regular and shorter intervals to determine the need for booster vaccine doses. The test method employed is economical and well suited to such evaluations.

Establishment and characterization of a cell line from the mosquito Culex (Culex) theileri (Diptera: Culicidae) and its susceptibility to infection with arboviruses.

The mosquito Culex (Culex) theileri Theobald is probably the major vector of Rift Valley fever (RVF) virus during epizootics on the inland plateau of southern Africa. A cell line was established from the tissues of this mosquito species to facilitate laboratory studies on RVF and other arboviruses. After temperature pretreatment, egg rafts from a laboratory colony were disinfected, mechanically disrupted, and grown in preconditioned Eagles minimum essential medium supplemented with 10% fetal calf serum. The cells were characterized by their protein content, karyology, and their susceptibility to three arboviruses. Although high infectivity titers were obtained, no cytopathic effects were produced by these viruses. The cell line was shown to be free from latent viruses. These results show that this cell line may be employed fruitfully in arbovirus studies.

Nosocomial rotavirus infections in a South African hospital nursery for white infants.

Rotavirus infections in a nursery ward for newborn white infants in South Africa do not apparently differ from those encountered in other temperate countries. Neonates began to excrete rotavirus as early as 2 days after birth; a peak incidence of excretion occurred during the winter months. After a period of 4 years the incidence of excretion remained virtually unchanged. Rotaviruses also colonized and spread very rapidly in a newly commissioned nursery ward, possibly via respiratory infections. Passively acquired antibodies had no influence on the prevention of infections but may play a role in the suppression of clinical symptoms since none of the newborn population studied developed gastro-enteritis. The immunosorbent electron microscopy technique used to detect rotaviruses in this study yielded results that compared well with results obtained previously by other techniques.